The invention concerns a laser nozzle usable for laser-beam cutting with the internal movable element comprising a skirt for concentrating the cutting gas in the cutting kerf, as well as a laser-cutting method using said nozzle, a laser focusing head on which said nozzle is fixed, and a laser cutting installation able to perform said method.
Laser-beam cutting requires the use of a nozzle, generally made from copper, the effect of which is to channel the gas and allow the laser beam to pass.
The nozzles typically have diameters of their outlet orifice of between 0.5 and 3 mm for a working distance of between 0.6 and 2 mm.
In order to enable cutting, it is necessary to use high pressures, generally of several bar, in the focusing head in order to enable the gas to enter the kerf so as to drive out the molten metal.
However, a large part of the gas used, typically between 50% and 90%, has no action on the cutting process, that is to say on the expulsion of the molten metal, since it starts on the sides of the cutting kerf.
These gas losses are in fact due to the enormous difference between the cross-section of flow of the nozzle orifice and the size of the focal spot. Thus, by way of indication, the cross-section of flow of a nozzle with an outlet orifice with a diameter of 1.5 mm is 25 times greater than the cross-section of the focal spot created by the laser beam passing through this nozzle.
However, if an insufficient proportion of gas is used, cutting defects then appear, in particular adherent burrs and/or traces of oxidation.
Attempting to remedy this by reducing the diameter of the nozzle orifice is not ideal since the risk is then taken of having the laser beam striking the inside of the nozzle and damaging it, which moreover also impairs the cutting quality and/or the performances.
There exist moreover a certain number of documents proposing various solutions for attempting to assist the entry of gas into the kerf, for example EP-A-1669159, JP-A-62006790 and JP-A-61037393.
However, none of these solutions is truly ideal since there is often a complex architecture to use, with a size greater than that of a conventional nozzle, and/or having limited efficacy.
Some solutions propose in particular to use a laser nozzle comprising a movable element, the end of which is pressed by a spring against the surface of the part to be cut in order to assist the injection of the cutting gas into the kerf.
The major drawback of these solutions lies in the fact that the force exerted by the spring in the direction of the metal sheet, added to the pressure of the cutting gas, leads the movable element to exert a high force on the sheet to be cut. The result is a risk of deformation, scratching or even driving of the metal sheet, which is in general simply placed on the table of the industrial cutting machine.
Furthermore, the movable element in contact with the metal sheet is liable to be damaged by burrs, splashing or obstacles forming reliefs on the surface of the sheet. This reduces the service life of the nozzle and impairs the productivity of the method.
To attempt to remedy this, the document JP-A-7251287 proposes a laser nozzle comprising a movable element and a spring exerting a force tending to press said element onto the sheet. Furthermore, JP-A-7251287 proposes using the force exerted by the cutting gas delivered in the nozzle and escaping under the surface of said element situated opposite the sheet to be cut in order to cause a slight distancing of the movable element with respect to the sheet.
However, this solution continues to pose certain problems.